Re: [Qemu-devel] [PATCH v3 3/4] x86: fill high bits of mtrr mask

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eduardo Habkost (ehabkost@redhat.com) wrote:
> On Tue, Jul 05, 2016 at 08:03:17PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Fill the bits between 51..number-of-physical-address-bits in the
> > MTRR_PHYSMASKn variable range mtrr masks so that they're consistent
> > in the migration stream irrespective of the physical address space
> > of the source VM in a migration.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
> [...]
> > @@ -2084,6 +2085,28 @@ static int kvm_get_msrs(X86CPU *cpu)
> >      }
> >  
> >      assert(ret == cpu->kvm_msr_buf->nmsrs);
> > +    /*
> > +     * MTRR masks: Each mask consists of 5 parts
> > +     * a  10..0: must be zero
> > +     * b  11   : valid bit
> > +     * c n-1.12: actual mask bits
> > +     * d  51..n: reserved must be zero
> > +     * e  63.52: reserved must be zero
> > +     *
> > +     * 'n' is the number of physical bits supported by the CPU and is
> > +     * apparently always <= 52.   We know our 'n' but don't know what
> > +     * the destinations 'n' is; it might be smaller, in which case
> > +     * it masks (c) on loading. It might be larger, in which case
> > +     * we fill 'd' so that d..c is consistent irrespetive of the 'n'
> > +     * we're migrating to.
> > +     */
> > +    if (cpu->fill_mtrr_mask && cpu->phys_bits < TARGET_PHYS_ADDR_SPACE_BITS) {
> 
> As we already ensure phys_bits <= TARGET_PHYS_ADDR_SPACE_BITS in
> patch 1/4, what about just doing this:
> 
>   assert(cpu->phys_bits <= TARGET_PHYS_ADDR_SPACE_BITS)
> 
> 
> > +        mtrr_top_bits = MAKE_64BIT_MASK(cpu->phys_bits,
> > +                                TARGET_PHYS_ADDR_SPACE_BITS - cpu->phys_bits);
> 
> What's the actual meaning of TARGET_PHYS_ADDR_SPACE_BITS? Can it
> ever change in the future?  Should a change in
> TARGET_PHYS_ADDR_SPACE_BITS really change the migration format?
> 
> To make sure we won't have any surprises if
> TARGET_PHYS_ADDR_SPACE_BITS change, I would change the code to:
> 
>  QEMU_BUILD_BUG_ON(TARGET_PHYS_ADDR_SPACE_BITS > 52);
>  assert(cpu->phs_bits <= TARGET_PHYS_ADDR_SPACE_BITS);
>  mtrr_top_bits = MAKE_64BIT_MASK(cpu->phys_bits, 52 - cpu->phys_bits);

Done.

All limits tend to stretch with time, so no it wouldn't surprise me if that
happened some day.

Dave

> 
> 
> > +    } else {
> > +        mtrr_top_bits = 0;
> > +    }
> > +
> >      for (i = 0; i < ret; i++) {
> >          uint32_t index = msrs[i].index;
> >          switch (index) {
> > @@ -2279,7 +2302,8 @@ static int kvm_get_msrs(X86CPU *cpu)
> >              break;
> >          case MSR_MTRRphysBase(0) ... MSR_MTRRphysMask(MSR_MTRRcap_VCNT - 1):
> >              if (index & 1) {
> > -                env->mtrr_var[MSR_MTRRphysIndex(index)].mask = msrs[i].data;
> > +                env->mtrr_var[MSR_MTRRphysIndex(index)].mask = msrs[i].data |
> > +                                                               mtrr_top_bits;
> >              } else {
> >                  env->mtrr_var[MSR_MTRRphysIndex(index)].base = msrs[i].data;
> >              }
> > -- 
> > 2.7.4
> > 
> 
> -- 
> Eduardo
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK